10 Answers
10

There are trees which are capable of killing large animals, in fact, only they don't eat them afterwards. But it's a start.

Pisonia brunoniana is known as the birdcatcher tree for a reason. The seed pods of this tree are coated with a mucus which traps insects - even this alone could evolve into carnivory, but wait till you hear what happens next. These trapped insects entice seabirds to come and feed, but as they do, they get covered in seeds.

These seeds are very sticky. As more and more of them latch onto the bird, they weigh it down, and clog up its plumage. Unable to fly, the helpless victims later die of starvation. Birds of prey, attracted by the corpses, often too get covered in the vicious seeds of the birdcatcher tree, and they too later starve.

So, how can we work with this to make a carnivorous tree? Let's have the sticky sap coat the tree's bark all over. Insects of course will get stuck in this all the time, and they shall be slowly digested by enzymes in the sap. While this happening, birds will come to eat the insects and also get stuck to the trunk. Any bird which tries to perch on a
branch will get stuck, as will the likes of monkeys, squirrels and other arboreal critters who mistake it for a normal tree.

The mucus/sap would be transparent, so that the killer tree would resemble a conventional one in all respects - aggressive mimicry. You could also have the sticky substance coat the roots too, to snag any passing soil fauna.

$\begingroup$I don't know that the transparency of the sap would matter very much. It seems like being coated in dead animals would be a bit of a giveaway.$\endgroup$
– WildcardMay 11 at 17:22

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$\begingroup$@Wildcard Depends how much animals there are. Unless you were up close, you wouldn't see the insects, and a few birds (especially if some were still stuck in perching pose) could easily not be noticed, considering that the animals will either be flying or leaping from tree to tree.$\endgroup$
– SealBoiMay 11 at 17:39

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$\begingroup$@Wildcard: I don’t know about you, but I had to stare at the answer’s photo quite hard before I found the bird in it.$\endgroup$
– Peter LeFanu LumsdaineMay 11 at 18:26

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$\begingroup$How does the digested food become part of the tree? Would it not just fall of the branches once a sufficient amount had been digested? (It's not a stretch of the imagination to have it sink into the soil and be absorbed by the roots, but there might be problems there.)$\endgroup$
– A Lambent EyeMay 11 at 18:50

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$\begingroup$@ALambentEye Well, the bugs would be small enough to be engulfed by the sap, and it wouldn't be an issue underground. For birds and arboreal tetrapods, you could have the tree detect the disturbance and start secreting extra sap to swallow the animal.$\endgroup$
– SealBoiMay 11 at 19:29

Rule 1: Don't feed like a tiger, feed like an alligator.

Alligators (and several other animals) eat only rarely: they can sustain themselves on just a single large prey animal every year, or even every two years, because they move slowly and don't use up much energy. Trees are great at moving slowly and conserving energy! That means that your particular tree doesn't need to hunt, it just needs to make sure an animal dies nearby often enough to have a few meals every once in a while. Once you've settled that, I propose one of two approaches, both of which allow for a totally regular tree, save for some small differences that still meet all your requirements.

Pit-y The Fool

Pit traps are an ancient human tradition, are used by antlion larvae, and, naturally, are used by pitcher plants. Making a pitcher plant tree-sized is tricky, since hollow structures aren't as stable, so instead, your tree grows over a pit trap. Its roots encircle a large, underground hollow, which it fills with a similar digestive fluid to pitcher plants —if we want— or just plain old water, if we don't. On the surface, its roots make a fine web that covers the otherwise open pit. The tree's own leaves fall and accumulate on top of it, disguising the unsafe ground. Along comes an animal, it steps on the leaves, and plunges down into the pit below, with no way to escape. There, it either drowns or starves, and its body either is dissolved by the digestive fluids or decomposes to nourish the soil the tree grows in.

When in doubt, violate the Geneva Convention

If you're a person with a pollen allergy, you may believe that trees are already nearly killing you...

And that's when they aren't even trying. Your carnivorous trees are ordinary trees in every respect, but their pollen is toxic (there is prior art here with the death camas). Once a year, in a forest of your carnivorous trees, a vast cloud of chemically lethal pollen boils off of your trees, killing hundreds of animals that have made the area their home. Of course, you would think that this would end up eliminating local populations to nothing, but in fact, the death pollen tree has developed a precarious balance, where its pollen is just toxic enough to only kill the old, infirm, or sick animals in its groves, while leaving the young and healthy ill for only a day or two, a sort of yearly reaping. In this way, it cultivates a very healthy, thriving ecosystem of animals, by making sure that only ones of reproductive age and viability live in its environs.

If you want to make them a little more literally carnivorous, take a page out of the book of Ophiocordyceps unilateralis, and have the toxin compel the dying animals to seek out the trunks of the trees that killed them, to slowly expire among their roots. This could be done with either hand-wavey toxins, or by saying that the pollen is lethal because of a symbiotic relationship with a bacteria that induces that behavior.

$\begingroup$Rather than trying to adapt it to only affect the old, sick or very young, you could instead say it only affects those who can’t get out of the way fast enough. To give a comparison, animals rarely die in forrest fires, it is only usually the old, sick, injured or very young as they can’t escape the flames fast enough. In this case, rather than escaping flames, animals are fleeing to escape pollen. So, whilst the young healthy adults are fast enough to escape, allowing the species’s survival, a small population will still be killed.$\endgroup$
– Liam MorrisMay 11 at 21:27

$\begingroup$Another possible alternative, rather than having the toxic pollen affect a whole area, have the pollen be in pods or sacs that only release when disturbed (think Alien: Covenant puff spores). This way you don't need to kill hundreds of animals that will never make it back to the tree to be fed upon.$\endgroup$
– David KMay 13 at 15:44

The tree has a commensal relationship with large predators. The predators use the tree as a refuge to eat their kills safely. The tree gets the leftovers. The tree has hollows in which scraps and offal land, and from there the tree puts forth adventitious roots. The tree exudes pyrethrins, poisoning the meat for flies and so the tree gets the fly larvae too. The pyrethrins are good for the large predator - they smear on its coat and kill fleas and ectoparasites.

Plants generally reproduce without physical contact - they have pollen. There would be a hurdle for these trees - how to get to adequate size to shelter leopards and start the meat coming? You could posit and earlier stage in the life cycle where the tree uses photosynthesis, but why give that up as an adult?

We will take a page from the playbook of the strangler fig.

The plant intermittently makes meaty fruits that smell great to leopards. The leopard poops out the seeds near trees it likes. The carnivorous plant wends its way across the ground to take advantage of carrion dropped from the tree and then climbs the tree as a tiny vine. Gradually it encompasses this tree, and becomes the tree.

I'd suggest pitcher plant style, the stem is hollow and full of sweet nectar that attracts small creatures. There are arguments about whether pitcher plants are strictly carnivorous as some suggest other options, but for the purposes of this question, we'll assume they are carnivorous.

Since the requirement to not move on its own accord prohibits various entanglement or venus fly trap style options, pitcher is probably the best way to go.

A tree with a hollow trunk, full of water containing a mild anaesthetic or sedative, this causes trapped creatures not to struggle too hard to escape which risks damaging the tree.

Most carnivorous plants are primarily plants, whatever additional nutrients they gain are just that, additional, they're capable of surviving without them but it helps. This means that all the normal aspects of a plant are present; roots, leaves, flowers, seeds. Your choice of pollination mechanism, whether insect, bird, wind or otherwise.

Consider perhaps something like an oak tree, the acorns on the outer branches are good eating for squirrels, and come the season they appear in vast numbers, but sometimes the acorns only appear on the inner branches, over the trap, and these ones are drugged. Beware little squirrel, when and which acorns you try to take. There's no way out if you fall in.

Perhaps the most obvious starting place, the venus flytrap is a plant with “jaws” that slam shut like a bear trap when the hairs on the inside are moved. Typically it requires several movements within a certain time frame in order to trigger the jaws to shut (so the plant knows that the thing inside its jaws is a creature and not a leaf or debris). Once closed, the jaws fill with a digestive fluid which breaks down the prey.

Applying this to your tree, there may be giant jaws like those of a venus flytrap scattered around the base of the tree. The jaws would be connected to the tree via an underground stem which runs from the jaws to the base of the tree. The actual tree portion looks pretty normal, perhaps mimicking other trees via interbreeding or simply evolving to look like, for example, a common oak tree or a fir tree. This has been seen in both snakes and insects where one species evolves to look like another.

Sweet, Delicious, Poison

Your tree may evolve poisonous fruit, slowly killing any large animal that ate it. In fact, apple seeds have cyanide in them. Well, to be more specific, they have a compound in them called Amygdalin that breaks down into hydrogen cyanide.

Amygdalin is a part of the seeds' chemical defenses. It is harmless when intact, but when the seeds are damaged, chewed or digested, amygdalin degrades into hydrogen cyanide. This is very poisonous and even lethal in high doses.

The fruits of your trees could contain far more Amygdalin, perhaps the entire fruit is laced with it. When a creature bites down into the fruit, the compound would start to degrade into lethal amounts of hydrogen cyanide and kill the creature relatively quickly. The fruits should have some sweeteners in them as well to encourage an animal to chew and swallow the fruit, making the poison more effective (i would imagine, based on apple seeds, that Amygdalin would make the fruit very bitter. By adding sweeteners, such as high amounts of sugar, this could help to mask the taste).

Once dead, the animal would start to decompose, providing nutrients to the tree. Scavengers may also be killed indirectly by consuming part of the poisoned creature. By eating the cyanide filled carcass, the scavenger in turn would be poisoned and die, providing more nutrients to the tree.

All of a sudden this makes the story of Snow White eating a poisoned apple and dying make sense, and gives it a much darker tone.

You May Feel a Little Pinch

As one of your challenge conditions is to have the tree covered in “something that resembles leaves or needles”, i propose something similar to Nettles. These are brightly green coloured plants covered in hypodermic needles that inject toxins into a creature that touches them.

You could have something like pine needles, found on pine trees, which function in a similar way. These needles could inject a toxin into an animal that touches the tree, quickly killing it. It may be hydrogen cyanide, as mentioned earlier, but to spice things up a bit i’ll also add in neurotoxins. These are toxins that affect the nervous system, your trees could paralyse its prey or give it a heart attack, killing the creature. Again, it would slowly decompose and provide nutrients to the tree. Any creature that got near risks touching the tree and being killed as well.

Simply because i love the idea of the leaves from a willow tree triggering some kind of trap. When a creature passes through the vines, it causes the tree to release toxic chemicals to kill said creature, perhaps in the form of pollen or some kind of gas or mist released from the tree.

The tree may have some internal requirements to trigger the release of its poison. For example, a small section the leaves parting and closing again with no other movement for a short period of time. This would prevent premature triggering due to the wind or debris. The leaves may also only grow a certain length, allowing very small animals to pass underneath and not trigger the trap (as it may be more effort than its worth to poison a single squirrel, for example.) However, larger animals would still trigger it as they are too big to go under the leaves.

Also, quick reference to Nausicaä, Valley of the Wind

Whilst not carnivorous trees, the plants release a toxic spore which kills any creature that inhales it. This idea could easily be translated to create carnivorous trees.

$\begingroup$This is very good. The neurotoxin could induce paralysis of voluntary muscles, leaving heartbeat and breath. Once the victim is disabled, the plant can slowly move more branches/leaves onto (and into) it. The plant might even provide sugar and water for the victim's bloodstream to prolong its life, even as it pulls out minerals, forming a temporary symbiosis, until the plant had removed so much that the victim goes into cardiac failure. That a sentient victim could be aware of its slow death, but unable to move a (voluntary) muscle to stop it, would make it all the more chilling.$\endgroup$
– Monty HarderMay 13 at 20:25

$\begingroup$@MontyHarder You want to know the scariest thing? Things like that already exist. Cordyceps is a fungi that releases spores which get inhaled by ants. The fungus then works its way to the ant’s brain, taking control of it and forcing it to climb as high as it can. Meanwhile, the fungus is breaking down all non-essential components of the ant, slowly killing it. Once the ant has climbed up a plant stem, the fungus forces the ant to latch its mandibles into the stem and locks them in place. -continued-$\endgroup$
– Liam MorrisMay 13 at 21:22

$\begingroup$The fungus then continues breaking down the rest of the ant’s internal components before a stem erupts out of its head and spreads its spores, continuing the cycle. A game called “The Last of Us” focuses on the idea of a mutated version of this fungus, creating zombie-like humans.$\endgroup$
– Liam MorrisMay 13 at 21:25

Like Liam Morris, I have always liked the idea of willow trees that trap prey. My mechanism of choice would be that the hanging leaves and branches are covered in a stringy sap that works like a spider's web to trap small animals. The more the animal struggles the more enmeshed they become and the tree then grows tendrils into the trapped prey to extract nutrients.

While the trees do not voluntarily move, the additional nutrients they gather from trapped animals could allow them to grow tendrils quickly and begin extracting nutrients within minutes of the animal being trapped. The sap would likely need to have some sort of narcotic effect so that trapped animals do not continue struggling. If the pollen has a similar effect it would serve the tree by causing nearby animals to be careless around it and more likely to be trapped. The effects of this sap and it's medicinal (or recreational) value could be useful to a plot or side plot, if you want to go there.

The tree stays disguised as a normal willow because branches with trapped prey will naturally sag downward and inward due to the weight of the prey, concealing them inside the "tent" created by the exterior foliage. This combined with continually growing new leaves around the outside leaves the tree appearing to be a normal willow unless you know exactly what to look for. Strip away that exterior tent of foliage and the scene would be a macabre one with numerous animals in various states of decay and a pile of bones on the ground below.

$\begingroup$This reminds me of a games series called Sorcery!, originally a series of Fighting Fantasy books by Steve Jackson. In Sorcery!, there is what appears to be an ordinary field of long grass. However, this grass is carnivorous, it wraps itself around its prey, strangling them to death. The most interesting thing is how it indirectly lures its prey. When humans are killed and turned into plant food, gold coins, jewels, weapons etc are left behind. This unattended treasure attracts other humans, continuing the cycle. You may also draw comparisons to the flower field in the Wizard of Oz.$\endgroup$
– Liam MorrisMay 12 at 9:21

I like the image of a Willow tree, but I'm picturing a more pro-active version that has branches that work like a jellyfish's tentacles (search cnidoblasts). if an animal contacts them they reflexively trigger with a similar hair mechanism to a Venus fly trap, binding and coiling the prey, perhaps injecting a sedative, with the whole branch contracting to bring it closer to the trunk.

It might be fun to add a symbiotic relationship to draw in prey.

I'm thinking a cloud of little bio-luminescent fairy-flies that feed off sap from the trunk of the tree but are too light to trigger any of the stingers. At night their light attracts in other larger animals looking for prey. Make the light blue and you have a Willow that Weeps for it's prey.

This doesn't fill the challenge criteria, but a giant frog with a sticky tongue would be a cool base.

It would have a metabolism like a large snake, slowly digesting its prey. It would have evolved to mimic bushes in the surroundings, maybe just hiding within them and evolving camouflage features like coloration and patterning, later bushy shapes on the head and such.

It would sit quiet for months, then suddenly opening its mouth to attack an animal (maybe human) with a giant 20-foot tongue, retracting the prey into its mouth in a split second. For extra effect, you could have it needing to chew its prey, but trying to keep it quiet
when it hears movement around.

$\begingroup$If you make a sketch of it, I'll gladly give you +1 for creativity.$\endgroup$
– A Lambent EyeMay 13 at 15:52

$\begingroup$@ALambentEye I'd need an animation of the attack, plus people wandering around the forest wondering what the faint chewing sound that stops when you get close is ;)$\endgroup$
– JollyJokerMay 13 at 19:33

Not quite a tree, but there is a bush that does something similar - it initially evolved exposed roots above-ground to stop herbivores (namely sheep) from reaching it. It then evolved spines on those roots to better stop sheep - it catches their wool, and they're then trapped, and starve, turning into tasty nutrients right over the roots.

The bush just needs to evolve actively trying to attract the sheep to become 'truly' carnivorous.

$\begingroup$Welcome to the Worldbuilding Stack Exchange! Should you find the time to do so, it would be good to add the name and perhaps some pictures of the plant.$\endgroup$
– A Lambent EyeMay 14 at 5:49

Huge, dark tree, that likes to grow amidst other huge dark trees. From below, it looks like any of the others, but in truth it grows like a bent-down rod(back to a few meters above ground ) ending in one row of wide horizontal branches (though hiding it well in a thicket of leaves) forming an umbrella-like structure, spring loaded to snap shut. The seeming lichen hanging down are the triggers. A critter passing below fits the prey profile? the umbrella snaps shut around the victim, and the whole tree springs straight, yanking the prey (prey size determined by tree size, adult trees swallow elephants every few years) into the air. Thorns and spiny protrusions will inject highly corrosive substances deep into the victim, who proceeds to trickle-down the tree over the next few days. Then begins the growing and reloading, taking months.